Breathable, heat-sealable composite polymeric film

ABSTRACT

A breathable, heat-sealable composite film comprising first and second layers of polymeric material wherein (i) the polymeric material of the first layer comprises copolyester comprising monomeric units derived from one or more diol(s); one or more dicarboxylic acid(s); and one or more poly(alkylene oxide) glycol(s); and (ii) the second layer is a heat-sealable polymeric layer, and a process for the production thereof.

This application is the National Stage filing of PCT Application No.PCT/GB2006/000086, filed 11 Jan. 2006, which claims priority of UKApplication No. GB0500906.3, filed 17 Jan. 2005.

This invention relates to breathable polymeric film, and a process forthe production thereof.

BACKGROUND OF THE INVENTION

Plastic film, particularly polyolefin film, has long been used in thepackaging of fresh-cut flowers and vegetables. The primary function ofsuch packaging film is to provide a protective seal against insects,bacteria and air-borne contaminants.

Nevertheless, fresh-cut flowers and vegetables have a limited shelf lifedue to fundamental changes in their biochemistry. Whilst in soil,photosynthesis allows the plant to grow and expire oxygen. However,after harvesting, the plant no longer converts sunlight and carbondioxide into carbohydrates and oxygen, but uses its stores ofcarbohydrate to excrete water and carbon dioxide. For plants that arewrapped in plastic packaging, the accumulation of trapped water in thepackaging leads to fungal growths and rotting of the fresh food. A studyof volatiles produced by fruits and vegetables has been made by P.M.A.Toivonen (“Non-ethylene, non-respiratory volatiles in harvested fruitsand vegetables; their occurrence, biological activity and control”,Postharvest Biology and Technology 12 (1997) 109-125). Plastic packagingfilm, particularly polyolefin film, acts as a water barrier and canactually make this situation worse by trapping excreted water within thepackage. Vegetables having a high moisture content, such as broccoli andmushrooms, are particularly susceptible. It would be desirable toprovide packaging which not only provides a barrier to insects, bacteriaand air-borne contaminants but which is also breathable to water (inother words, a film which would allow water to freely pass out of thepackaging), thereby increasing the shelf life of the contents bydelaying the onset of microbial and fungal growth.

There already exist a number of commercially available breathable films.Several companies market polyolefin films which have been voided usinginorganic fillers thereby allowing easier passage of water. However,such films are not normally transparent and therefore have adisadvantage from a marketing standpoint. Other commercially availablefilms based on thermoplastic elastomers (TPEs) provide excellentbreathability but are expensive for packaging applications.

Another approach to the manufacture of breathable packaging has been toperforate the film in order to allow egress of excreted gas. The numberof perforations per unit area and the perforation diameter may beadjusted depending on the goods to be packaged and the respiration ratethereof. The disadvantage of this approach is that the perforationsexpose the packaged goods to insects, bacteria and air-bornecontaminants. Optionally heat-sealable composite films comprising aperforated layer are disclosed in WO-01/92000-A, GB-2355956-A,EP-0358461-A; EP-0178218-A; US-2002/0187694-A; JP-A-06/219465-A;JP-06/165636-A; and JP-54/117582-A. Co-pending applicationPCT/GB2004/003119 describes a breathable, heat-sealable, composite filmcomprising a perforated polymeric substrate layer and an unperforatedbarrier layer coated on a surface of the substrate.

It is an object of this invention to provide a breathable, heat-sealablepackaging film which allows egress of moisture and excreted gas,particularly moisture, from the packaged product but which provides aphysical barrier to insects, bacteria and airborne contaminants,particularly wherein the film is optically clear, and particularlywherein the manufacturing of the film can be achieved more efficientlyand economically.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a breathable,heat-sealable, composite film comprising first and second layers ofpolymeric material wherein:

-   (i) the polymeric material of the first layer comprises copolyester    comprising monomeric units derived from one or more diol(s); one or    more dicarboxylic acid(s); and one or more poly(alkylene    oxide)glycol(s); and-   (ii) the second layer is a heat-sealable polymeric layer.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “breathable” means that the composite film ofthe present invention is gas-permeable, particularly to water vapour andpreferably also to oxygen.

Preferably, the composite film has a water vapour transmission rate(WVTR) of at least 60 g/m²/day, preferably at least 80 g/m²/day,preferably at least 90 g/m²/day, preferably at least 100 g/m²/day,preferably at least 115 g/m²/day, preferably at least 130 g/m²/day,preferably at least 140 g/m²/day, and more preferably at least 150g/m²/day. The WVTR may be adjusted by variation of the film compositiondepending on the goods to be packaged and the respiration rate thereof.

In a preferred embodiment, the composite film is formed by coextrusion,with the first and second layers being coextruded according toconventional techniques in the art. In an alternative embodiment, theheat-sealable (second) layer is coated onto a substrate of the firstlayer as defined herein.

The composite film is a self-supporting film or sheet by which is meanta film or sheet capable of independent existence in the absence of asupporting base. The composite film is preferably uniaxially orbiaxially oriented, preferably biaxially oriented.

The polymeric material of the first layer is preferably non-elastomeric,by which is meant that it does not have the elastic properties exhibitedby natural rubber (such as resumption of the original shape when adeforming force is removed). The first layer preferably has anelongation to break of less than 250%, preferably less than 200%.

Suitable dicarboxylic acids for the copolyester of the first layerinclude aromatic acids such as terephthalic acid, isophthalic acid,phthalic acid and 2,5-, 2,6- or 2,7-naphthalene dicarboxylic acid, aswell as aliphatic dicarboxylic acids such as succinic acid, sebacicacid, adipic acid and azelaic acid. Cycloaliphatic dicarboxylic acidsmay also be used. Other suitable dicarboxylic acids include4,4′-diphenyldicarboxylic acid and hexahydro-terephthalic acid.Sulphonated aromatic dicarboxylic acids in which a sulfonate group isattached to the aromatic nucleus may also be used, and preferably thesulfonate group of such a sulfomonomer is a sulfonic acid salt,preferably a sulfonic acid salt of a Group I or Group II metal,preferably lithium, sodium or potassium, more preferably sodium.Ammonium salts may also be used. The sulphonated aromatic dicarboxylicacid may be selected from any suitable aromatic dicarboxylic acid, e.g.terephthalic acid, isophthalic acid, phthalic acid, 2,5-, 2,6- or2,7-naphthalenedicarboxylic acid. However, preferably the aromaticdicarboxylic acid of the sulfomonomer is isophthalic acid. Preferredsulfomonomers are 5-sodium sulpho-isophthalic acid and 4-sodiumsulpho-isophthalic acid. The term “dicarboxylic acids” as used hereinincludes acid equivalents of dicarboxylic acids having two functionalcarboxyl groups which perform substantially like dicarboxylic acids inreaction with glycols and diols in forming copolyesters, includingesters and ester-forming derivatives, such as acid halides andanhydrides. The dicarboxylic acids preferably have a low molecularweight (i.e. a molecular weight of less than about 300, which refers tothe molecular weight of the acid and not to its equivalent ester orester-forming derivative).

The copolyester of the first layer preferably comprises at least onearomatic acid, particularly terephthalic or naphthalene dicarboxylicacids, and particularly terephthalic acid. In a first embodiment, thedicarboxylic acid component comprises only one dicarboxylic acid. In asecond embodiment, the dicarboxylic acid component comprises a firstaromatic dicarboxylic acid preferably terephthalic acid) and a seconddicarboxylic acid. The second dicarboxylic acid may be selected fromaliphatic dicarboxylic acids such as succinic acid, sebacic acid, adipicacid or azelaic acid, and in one embodiment the second dicarboxylic acidis azelaic acid. Alternatively, the second carboxylic acid may beselected from sulphonated aromatic dicarboxylic acids such as sodiumsulpho-isophthalic acid. In these embodiments, the first dicarboxylicacid is preferably present in an amount of 50 to 100%, (preferably 60 to99 mol %, preferably 70 to 95 mol %, preferably 80 to 90 mol %) of thetotal acid component, and the second dicarboxylic acid is preferablypresent in an amount of 0 to 50 mol % (preferably 1 to 40 mol %,preferably 5 to 30 mol %, preferably 5 to 15 mol %) of the total acidcomponent. Where the second dicarboxylic acid is a sulphonated aromaticdicarboxylic acid, it is preferably present in amounts of 0 to 15%(preferably 0 to 10%, preferably 1 to 10%, preferably 2 to 6 mol %) ofthe total acid component. Preferably, the copolyester of the first layercomprises only one type of non-sulphonated aromatic dicarboxylic acidresidue, and in one embodiment only one type of aromatic dicarboxylicresidue. In one embodiment, at least 90 mol %, preferably at least 95mol %, preferably at least 98 mol %, and preferably at least 99 mol % ofthe dicarboxylic acid fraction is made up of one type of non-sulphonatedaromatic dicarboxylic acid.

Suitable diols for the copolyester of the first layer are low molecularweight diols (i.e having a molecular weight below about 250) includingacyclic, alicyclic and aromatic dihydroxy compounds. Preferred compoundsare diols with 2-15 carbon atoms such as ethylene, propylene,isobutylene, tetramethylene, 1,4-pentamethylene,2,2-dimethyltrimethylene, hexamethylene and decamethylene glycols,dihydroxycyclohexane, cyclohexane dimethanol, resorcinol, hydroquinone,1,5-dihydroxynaphthalene, etc. An aliphatic diol is preferred,especially acyclic aliphatic diols containing 2-8 carbon atoms,preferably ethylene glycol and 1,4-butanediol, more preferably ethyleneglycol. Cycloaliphatic (alicylic) glycols such as1,4-cyclohexanedimethanol (CHDM) can also be used. Equivalentester-forming derivatives of diols may be used in place of the diol. Theterm “low molecular weight diols” as used herein encompasses suchequivalent ester-forming derivatives; provided that the molecular weightrequirement pertains to the diol and not to its derivatives. Preferably,the copolyester of the first layer comprises only one type of lowmolecular weight diol residue. In one embodiment, at least 90 mol %,preferably at least 95 mol %, preferably at least 98 mol %, andpreferably at least 99 mol % of the low molecular weight diol fractionis made up of one type of low molecular weight diol. Preferably, the lowmolecular weight diol fraction present in the copolyester is in therange of 50 mol % or more, preferably 55 mol % or more, preferably inthe range of 60 to 90 mol %, and in one embodiment in the range of 60 to80 mol %, of the total glycol fraction of the copolyester.

Suitable poly(alkylene oxide) glycols for the copolyester of the firstlayer are preferably selected from C₂ to C₁₅, preferably C₂ to C₁₀,preferably C₂ to C₆ alkylene chains, and preferably selected frompolyethylene glycol (PEG), polypropylene glycol (PPG) andpoly(tetramethylene oxide) glycol (PTMO), preferably polyethyleneglycol. Ethylene oxide-terminated poly(propylene oxide) segments mayalso be used. Mixtures of poly(alkylene oxide) glycols can be used, butin a preferred embodiment the copolyester of the first layer comprisesonly one type of poly(alkylene oxide) glycol residue. The averagemolecular weight of a poly(alkylene oxide) glycol used in the presentinvention is preferably at least about 400 (and typically at least about1000), and preferably no more than 10,000, preferably no more than 4500,and in one embodiment no more than 2500. The amount of poly(alkyleneoxide) glycol present in a copolyester is preferably no more than 50 mol% of the glycol fraction of the copolyester, preferably no more than 45mol %, and preferably in the range of 10 to 40 mol %, and in oneembodiment in the range of 20 to 40 mol %.

Thus, in one embodiment, the copolyester of the first layer comprisesmonomer units derived from poly(alkylene oxide) glycol (preferably PEG),a low molecular weight aliphatic diol (preferably ethylene glycol), anaromatic dicarboxylic acid (preferably terephthalic acid) and a secondadditional dicarboxylic acid selected from aliphatic dicarboxylicacid(s) and sulphonated aromatic dicarboxylic acid(s).

The copolyesters used in the present invention may be described as block(segmented) copolymers comprising alternating random-length sequencesjoined by ester linkages having the general formula I:

wherein

-   -   R is the alkylene chain from the poly(alkylene oxide) glycol,        (preferably poly(ethylene oxide) glycol;    -   R₁ is derived from the low molecular weight diol(s) (preferably        ethylene glycol);    -   A is an aromatic ring (preferably phenyl or naphthyl, and more        preferably phenyl) derived from the aromatic dicarboxylic        acid(s) (preferably terephthalic acid (TA), isophthalic acid        (IPA) or 2,6-naphthalene dicarboxylic acid, and more preferably        TA);    -   the ratio x:y is in the range of from about 3:1 to about 80:1        (and in one embodiment from greater than 40:1 to about 80:1);    -   y is preferably in the range of 1-3; and    -   n is the degree of polymerisation required to give the molecular        weights recited hereinabove, and is typically in the range of        14-28.

In one embodiment x is in the range of 3-80, and in one embodiment fromgreater than 40 to about 200, and in a further embodiment from greaterthan 40 to about 80.

Where more than one low molecular weight diol and/or dicarboxylic acidis/are present, the number of different alternating blocks increases.

Suitable copolyesters for use in the first layer of the presentinvention preferably have a glass transition temperature (Tg) of atleast 0° C., preferably at least 15° C., preferably at least 30° C., andare preferably no more than 60° C., preferably no more than 55° C. Thus,in a preferred embodiment, the Tg of the copolyesters are in the rangeof from about 30° C. to about 60° C.

Preparation of the Copolyesters is Readily Achieved by ConventionalMethods Well-Known in the art. Typically, a conventional esterinterchange reaction is utilised, comprising heating an ester of thedicarboxylic acid, for example the dimethyl ester of terephthalic acid,with the poly(alkylene oxide)glycol and a molar excess of the lowmolecular weight diol in the presence of a catalyst at elevatedtemperature (typically in the range of about 150 to 260° C.), followedby distilling off the alcohol (e.g. methanol) formed by the interchangereaction. Alternatively, the direct esterification route may be used,which comprises heating at least one aromatic acid, for exampleterephthalic acid, with the poly(alkylene oxide) glycol and a molarexcess of the low molecular weight diol in the presence of a catalyst atelevated temperature (typically in the range of about 150 to 260° C.)followed by distilling off the water formed by the esterificationreaction. The next step in the synthesis is a conventionalpolycondensation step to increase the molecular weight of the polymer tothe desired level. Preferably, an anti-foaming agent (such as a siliconeoil, for example Dow Corning's DC1510) is utilised in the synthesis inorder to reduce the risk of the autoclave becoming blocked withpre-polymer.

The polymeric material of the first layer preferably comprises greaterthan 50%, preferably at least 65%, preferably at least 80%, preferablyat least 95% and preferably at least 99% of the aforementionedcopolyester, the stated weights of copolyester being the percent byweight relative to the total amount of polymeric material in the firstlayer. Preferably the polymeric material of the first layer consistsessentially of the aforementioned copolyester.

The thickness of the first layer is typically from about 0.5 to 300 μm,preferably from 12 to about 200 μm and particularly from about 12 toabout 100 μm.

The polymeric material of the second layer is heat-sealable. Thepolymeric material of the second layer should soften to a sufficientextent that its viscosity becomes low enough to allow adequate wettingfor it to adhere to the surface to which it is being bonded. Preferably,the composite film exhibits a heat-seal strength to itself (i.e. bycontacting its heat-sealable second layers) of at least 300 g/25 mm,preferably from about 400 g/25 mm to about 1000 g/25 mm, and morepreferably from about 500 to about 850 g/25 mm. The polymeric materialof the first layer does not provide an effective heat-seal property, anddoes not exhibit a heat-seal strength to itself of at least 300 g/25 mm.

In one embodiment, hereinafter referred to as Embodiment A, the secondlayer may comprise any suitable conventional heat-sealable polymericmaterial and has a thickness such that it does not significantly impairthe breathable characteristics of the first layer. Preferably, the WVTRof the composite film comprising the first and second layer as definedherein should be no more than 50%, preferably no more than 35%,preferably no more than 20% and preferably no more than 10% lower thanthe WVTR of the first layer in isolation. With regard to the thicknessof the heat-sealable layer of Embodiment A, there is an inversecorrelation between the layer thickness and the WVTR of the compositefilm and, in general, as the thickness of the second layer doubles, theWVTR of the composite film halves. Suitable polymeric materials forEmbodiment A include heat-sealable polyesters. The polyester istypically a copolyester and is derived from a first dicarboxylic acidand a first low molecular weight diol, and optionally a seconddicarboxylic acid and/or second low molecular weight diol. Preferably,the copolyester comprises either a second dicarboxylic acid residue or asecond low molecular weight diol residue.

In an alternative embodiment, hereinafter referred to as Embodiment B,the polymeric material of the second layer comprises a copolyestersimilar to that of the first layer, but wherein the copolyester of thesecond layer differs by the incorporation of an additional comonomer,not present in the copolyester of the first layer, which impartsheat-sealability (as defined herein) to the second layer. The molarfraction of poly(alkylene oxide) glycol in the two copolyesters may bethe same or different, but is preferably substantially the same. Theadditional comonomer is incorporated into the copolyester at the expenseof the dicarboxylic acid fraction and/or the low molecular weight diolfraction, and in a preferred embodiment at the expense of either thedicarboxylic acid fraction or the low molecular weight diol fraction. Inone embodiment, the additional co-monomer is incorporated into thecopolyester at the expense of the dicarboxylic acid fraction. Suitablecomonomers include isophthalic acid and a low molecular weight aliphaticdiol (particularly a cycloaliphatic diol such as CHDM).

In one embodiment, hereinafter referred to as Embodiment A1, the secondlayer comprises a copolyester derived from an aliphatic glycol(preferably ethylene glycol), a first aromatic dicarboxylic acid(preferably terephthalic acid) and a second dicarboxylic acid(preferably isophthalic acid). Thus, a preferred copolyester is derivedfrom ethylene glycol, terephthalic acid and isophthalic acid. Thepreferred molar ratio of the first dicarboxylic acid (preferablyterephthalic acid) to the second dicarboxylic acid (preferablyisophthalic acid) is in the range of from 50:50 to 90:10, preferably inthe range from 65:35 to 85:15, and is preferably about 82:18.

In a further embodiment, hereinafter referred to as Embodiment A2, thesecond layer comprises a copolyester derived from an first diol(preferably an aliphatic diol, preferably ethylene glycol or butanediol, more preferably ethylene glycol) and a second diol (preferably acycloaliphatic diol, for instance 1,4-cyclohexanedimethanol) with one ormore, preferably one, dicarboxylic acid(s), preferably an aromaticdicarboxylic acid. Examples include copolyesters of terephthalic acidwith an aliphatic diol and a cycloaliphatic diol, especially ethyleneglycol and 1,4-cyclohexanedimethanol. The preferred molar ratios of thesecond diol (preferably cycloaliphatic) to the first diol (preferablyaliphatic) are in the range from 10:90 to 60:40, preferably in the rangefrom 20:80 to 40:60, and more preferably from 30:70 to 35:65. In apreferred embodiment this copolyester is a copolyester of terephthalicacid with about 33 mole % 1,4-cyclohexane dimethanol and about 67 mole %ethylene glycol. An example of such a polymer is PETG™6763 (Eastman)which comprises a copolyester of terephthalic acid, about 33%1,4-cyclohexane dimethanol and about 67% ethylene glycol and which isalways amorphous. In an alternative embodiment of the invention, thepolymer of the second layer may comprise butane diol in place ofethylene glycol.

In a further embodiment, hereinafter referred to as Embodiment A3, thesecond layer comprises an aromatic dicarboxylic acid and an aliphaticdicarboxylic acid. A preferred aromatic dicarboxylic acid isterephthalic acid. Preferred aliphatic dicarboxylic acids are selectedfrom sebacic acid, adipic acid and azelaic acid. The concentration ofthe aliphatic dicarboxylic acid present in the copolyester is preferablyin the range from 20 to 55, more preferably 30 to 50, and particularly35 to 45 mole % based on the dicarboxylic acid components of thecopolyester. Particularly preferred examples of such copolyesters are(i) copolyesters of azeleic acid and terephthalic acid with an aliphaticglycol, preferably ethylene glycol; (ii) copolyesters of adipic acid andterephthalic acid with an aliphatic glycol, preferably ethylene glycol;and (iii) copolyesters of sebacic acid and terephthalic acid with analiphatic glycol, preferably butylene glycol. Preferred polymers includea copolyester of sebacic acid/terephthalic acid/butylene glycol(preferably having the components in the relative molar ratios of45-55/55-45/100, more preferably 50/50/100) having a glass transitionpoint (T_(g)) of −40° C. and a melting point (T_(m)) of 117° C.), and acopolyester of azeleic acid/terephthalic acid/ethylene glycol(preferably having the components in the relative molar ratios of40-50/60-50/100, more preferably 45/55/100) having a T_(g) of −15° C.and a T_(m) of 150° C.

In a further alternative embodiment, hereinafter referred to asEmbodiment A4, the additional heat-sealable layer comprises an ethylenevinyl acetate (EVA). Suitable EVA polymers may be obtained from DuPontas Elvax resins. Typically, these resins have a vinyl acetate content inthe range of 9% to 40%, and typically 15% to 30%.

In one embodiment, hereinafter referred to as Embodiment B1, the secondlayer comprises a copolyester derived from a first aromatic dicarboxylicacid (preferably terephthalic acid), a low molecular weight aliphaticdiol (preferably ethylene glycol) and a poly(alkylene oxide) glycol, afurther dicarboxylic acid which imparts heat-sealability (preferably anaromatic dicarboxylic acid such as isophthalic acid). In thisembodiment, the poly(alkylene oxide)glycol is present in the amountsdescribed hereinabove in respect of the copolyester of the first layer.The first dicarboxylic acid is preferably present in amounts of from 50to 90 mol % of the total acid component. The heat-sealing dicarboxylicacid (preferably isophthalic acid) is preferably present in an amount offrom about of from 10 to about 50 mol % (preferably about 15 to 35 mol%, preferably about 18%) of the total dicarboxylic acid component. Apreferred copolyester for the heat-sealable copolyester of Embodiment B1is derived from poly(alkylene oxide)glycol, ethylene glycol,terephthalic acid and isophthalic acid. Optionally, the second layercopolyester of Embodiment B1 may also comprise a further dicarboxylicacid monomeric unit and this is selected from the “second dicarboxylicacid” defined hereinabove in respect of the copolyester of the firstlayer. If present in the heat-sealable copolyester of Embodiment B1,this “second dicarboxylic acid” is preferably selected from thesulphonated aromatic dicarboxylic acids (preferably sodiumsulpho-isophthalic acid) described hereinabove. Thus, the copolyesterwould then preferably be derived from poly(alkylene oxide) glycol,ethylene glycol, terephthalic acid, isophthalic acid and a sulponatedaromatic dicarboxylic acid (preferably sodium sulpho-isophthalic acid)as defined herein, in which the sulphonated acid is present in amountsof from about 1 to about 10 mol % (more typically about 1 to 6%) of thetotal acid component, and the isophthalic acid is present in amounts offrom about 10 to about 50 mol % the total acid component. Typically,however, said “second dicarboxylic acid” is not present in theheat-sealable copolyester of Embodiment B1.

In a further embodiment, hereinafter referred to as Embodiment B2, thesecond layer comprises a copolyester derived from a first dicarboxylicacid (preferably aromatic), a first low molecular weight diol(preferably an aliphatic diol, preferably ethylene glycol or butanediol, more preferably ethylene glycol) and a poly(alkylene oxide)glycol, and a second low molecular weight diol which impartsheat-sealability (preferably a cycloaliphatic diol, for instance1,4-cyclohexanedimethanol). Optionally, the “second dicarboxylic acid”defined hereinabove may also be present in the copolyester butpreferably only one dicarboxylic acid residue is present. Examples ofEmbodiment B2 include copolyesters of an aromatic dicarboxylic acid(preferably terephthalic acid) with a poly(alkylene oxide) glycol, analiphatic diol and a cycloaliphatic diol, especially ethylene glycol and1,4-cyclohexanedimethanol. The first low molecular weight diol ispreferably present in amounts of 40 to 90 mol % of the total glycolfraction, preferably in the range from 50 to 80 mol %. The second lowmolecular weight diol is preferably present in amounts of 10 to 40 mol %of the total glycol fraction. The poly(alkylene oxide) glycol ispreferably present in amounts of 10 to 40 mol % of the total glycolfraction.

The thickness of the second layer is preferably no more than 12 μm,preferably no more than 10 μm, preferably no more than 6 μm, andpreferably no more than 2 μm. The lower limit of thickness is typically0.05 μm, more typically 0.5 μm. Preferably, the thickness of the secondlayer is no more than 15% (preferably no more than 10% and preferably nomore than 5%) of the thickness of the first layer, particularly when thesecond layer is formed by co-extrusion.

Formation of the composite film is effected by conventional techniqueswell-known in the art. In general terms the process comprises the stepsof extruding one or more layers of molten polymeric material, quenchingthe extrudate and orienting the quenched extrudate in at least onedirection.

The composite film may be uniaxially oriented, but is preferablybiaxially oriented by drawing in two mutually perpendicular directionsin the plane of the film to achieve a satisfactory combination ofmechanical and physical properties. Orientation may be effected by anyprocess known in the art for producing an oriented film, for example atubular or flat film process. In the preferred flat film process, thefilm-forming polymeric material are extruded through a slot die andrapidly quenched upon a chilled casting drum to ensure that the polymeris quenched to the amorphous state. Orientation is then effected bystretching the quenched extrudate in at least one direction at atemperature above the glass transition temperature of the polymericmaterial. Sequential orientation may be effected by stretching a flat,quenched extrudate firstly in one direction, usually the longitudinaldirection, i.e. the forward direction through the film stretchingmachine, and then in the transverse direction. Forward stretching of theextrudate is conveniently effected over a set of rotating rolls orbetween two pairs of nip rolls, transverse stretching then beingeffected in a stenter apparatus. Alternatively, the cast film may bestretched simultaneously in both the forward and transverse directionsin a biaxial stenter. Stretching is effected to an extent determined bythe nature of the polymeric material, for example polyethyleneterephthalate is usually stretched so that the dimension of the orientedfilm is from 2 to 5, more preferably 2.5 to 4.5 times its originaldimension in the or each direction of stretching. Typically, stretchingis effected at temperatures in the range of 50 to 125° C. Greater drawratios (for example, up to about 8 times) may be used if orientation inonly one direction is required. It is not necessary to stretch equallyin the machine and transverse directions although this is preferred ifbalanced properties are desired.

A stretched film may be, and preferably is, dimensionally stabilised byheat-setting under dimensional restraint at a temperature above theglass transition temperature of the polymeric material but below themelting temperature thereof, to induce crystallisation of the polymer.In applications where film shrinkage is not of significant concern, thefilm may be heat set at relatively low temperatures or not at all. Onthe other hand, as the temperature at which the film is heat set isincreased, the tear resistance of the film may change. Thus, the actualheat set temperature and time will vary depending on the composition ofthe film but should not be selected so as to substantially degrade thetear resistant properties of the film. Within these constraints, a heatset temperature of about 135° to 250° C. is generally desirable, asdescribed in GB-A-838708.

Formation of the heat-sealable (second) layer and application thereof tothe first layer may be effected by conventional techniques, andtypically depends on the identity of the heat-sealable layer.Conventional techniques include casting or coating the heat-sealablesecond layer onto a preformed first layer, or by coextrusion of thefirst and second layers. In one embodiment, the formation of theheat-sealable (second) layer and the first layer is effected bycoextrusion, which would be suitable for Embodiments A1, A2, B1 and B2above. Other methods of forming the heat-sealable (second) layer includecoating the heat-sealable polymer onto the first layer, and thistechnique would be suitable for Embodiments A3 and A4 above. Coating maybe effected using any suitable coating technique, including gravure rollcoating, reverse roll coating, dip coating, bead coating,extrusion-coating, melt-coating or electrostatic spray coating. Coatingmay be conducted “off-line”, i.e. after any stretching and subsequentheat-setting employed during manufacture of the first layer, or“in-line”, i.e. wherein the coating step takes place before, during orbetween any stretching operation(s) employed. Preferably, coating isperformed in-line, and preferably between the forward and sidewaysstretches of a biaxial stretching operation (“inter-draw” coating).Prior to application of the heat-sealable (second) layer onto the firstlayer, the exposed surface of the first layer may, if desired, besubjected to a chemical or physical surface-modifying treatment toimprove the bond between that surface and the subsequently appliedlayer. For example, the exposed surface may be subjected to a highvoltage electrical stress accompanied by corona discharge.Alternatively, the first layer may be pretreated with an agent known inthe art to have a solvent or swelling action on the substrate, such as ahalogenated phenol dissolved in a common organic solvent e.g. a solutionof p-chloro-m-cresol, 2,4-dichlorophenol, 2,4,5- or2,4,6-trichlorophenol or 4-chlororesorcinol in acetone or methanol.

The composite film of the present invention preferably has an elongationto break of less than 250%, preferably less than 200%.

According to a further aspect of the present invention, there isprovided a process for producing a breathable, heat-sealable compositefilm, said process comprising:

-   (i) providing a first layer of polymeric material comprises    copolyester comprising monomeric units derived from one or more    diol(s); one or more dicarboxylic acid(s); and one or more    poly(alkylene oxide)glycol(s); and-   (ii) providing on a surface of the first layer a heat-sealable    second polymeric layer.

One or more of the layers of the polymeric film may conveniently containany of the additives conventionally employed in the manufacture ofpolymeric films. Thus, agents such as cross-linking agents, dyes,pigments, voiding agents, lubricants, anti-oxidants, radical scavengers,UV absorbers, thermal stabilisers, anti-blocking agents, surface activeagents, slip aids, optical brighteners, gloss improvers, prodegradents,viscosity modifiers and dispersion stabilisers may be incorporated asappropriate. In particular the composite film may comprise a particulatefiller which may, for example, be a particulate inorganic filler or anincompatible resin filler or a mixture of two or more such fillers. Suchfillers are well-known in the art. Particulate inorganic fillers includeconventional inorganic fillers, and particularly metal or metalloidoxides, such as alumina, silica and titania, calcined china clay andalkaline metal salts, such as the carbonates and sulphates of calciumand barium. Preferred particulate inorganic fillers include titaniumdioxide and silica. The inorganic filler is preferably finely-divided.The components of the composition of a layer may be mixed together in aconventional manner. For example, by mixing with the monomeric reactantsfrom which the layer polymer is derived, or the components may be mixedwith the polymer by tumble or dry blending or by compounding in anextruder, followed by cooling and, usually, commination into granules orchips. Masterbatching technology may also be employed.

In the preferred embodiment, the film of the present invention isoptically clear, preferably having a % of scattered visible light (haze)of <10%, preferably <6%, more preferably <5% and particularly <2%,measured according to the standard ASTM D 1003. Preferably, the totallight transmission (TLT) in the range of 400-800 nm is at least 75%,preferably at least 80%, and more preferably at least 90%, measuredaccording to the standard ASTM D 1003. In this embodiment, filler (ifpresent) is typically present in only small amounts, generally notexceeding 0.5% and preferably less than 0.2% by weight of the polymer ofa given layer.

The primary use of the film of the present invention is as a breathablefilm for packaging freshly cut plants such as flowers, vegetables, fruitand salads. The film described herein provides a physical barrier toentry of external contaminants, such as insects, bacteria and air-bornecontaminants, which would spoil the substance contained within thepackaging during transport and storage, while at the same time allowingegress of water vapour. The film may make up all or substantially all ofthe packaging for such products, or may comprise only part of the totalpackaging. For instance, the film may form a heat-sealed lid onto areceptacle in or on which the goods are disposed. The receptacle may bea tray such as a thermoformed tray or bowl, and may, for instance beformed of polyester, such as polyethylene terephthalate, or ofpolypropylene, polystyrene, or may be PVDC-coated. A film according tothe invention is particularly suitable for use as a lid on a receptaclemade from, for instance, polyester, polystyrene or polypropylene.Sealing of the receptacle is effected by techniques well-known to thoseskilled in the art. Once the contents to be packaged have beenintroduced into the receptacle, the heat-sealable film lid is placedupon the receptacle such that the heat-sealable layer of the film is incontact with the receptacle, and is affixed using temperature and/orpressure using conventional techniques and equipment. In an alternativeembodiment, the film is heat-sealed to itself to form substantially allof the packaging.

In an alternative embodiment, the film is heat-sealed to itself to formsubstantially all of the packaging. In this embodiment, the seal isprovided by heat-sealing a first portion of the film to a second portionof the film. Such seals are effected by conventional techniques andinclude “fin seals” and “overlap seals”, preferably fin seals. Once theproduct is placed within the film, the two portions of the film whichare to be bonded together are brought together with the heat sealablesurface of one film portion being in contact with the heat sealablesurface of the other film portion, and the heat-seal bond formed by theapplication of temperature and optionally pressure using conventionalequipment. The heat-seal bond is typically formed at temperatures in therange of about 120 to about 200° C., preferably about 140 to about 200°C.

The first surface of the substrate is the surface which is outermostwhen the film is used as such packaging, the second surface being thesurface which is innermost and faces the goods to be packaged.

In a further aspect, the present invention provides the use of a film asdescribed herein as a breathable film in the packaging of cut plant(s),for instance flowers or foodstuffs (including vegetables, fruits andsalads), particularly wherein said packaging comprises the film as alidding film and further comprises a receptacle.

In a further aspect, the present invention provides the use of a film asdescribed herein as packaging for cut plant(s), for instance flowers orfoodstuffs (including vegetables, fruits and salads), particularlywherein said packaging comprises the film as a lidding film and furthercomprises a receptacle, for the purpose of improving the breathabilityof said packaging and/or extending the shelf-life of said cut plants.

In a further aspect the invention provides a method of packaging cutplant(s), for instance flowers or foodstuffs (including vegetables,fruits and salads), said method comprising the step of providing a filmas described herein as at least part of said packaging, and particularlywherein said packaging comprises said film as a lidding film and furthercomprises a receptacle.

In a further aspect the invention provides a method of improving thebreathability of packaging of cut plant(s) and/or extending theshelf-life of said cut plants, wherein for instance said cut plant(s)are flowers or foodstuffs (including vegetables, fruits and salads),said method comprising the step of providing a film as described hereinas at least part of said packaging, and particularly wherein saidpackaging comprises said film as a lidding film and further comprises areceptacle.

In a further aspect, the invention provides a sealed containercomprising a receptacle containing cut plant(s), for instance flowers orfoodstuffs (including vegetables, fruits and salads), and a lid formedfrom a polymeric film as defined herein.

In a further aspect, the invention provides a packaged, sealed productcomprising cut plant(s), for instance flowers or foodstuffs (includingvegetables, fruits and salads), wherein the packaging which effects andforms the seal around the food product is a composite film as definedherein which is heat-sealed to itself.

The following test methods may be used to characterise the polymericfilm:

-   (i) Clarity of the film is evaluated by measuring total light    transmission (TLT) and haze (% of scattered transmitted visible    light) through the total thickness of the film using a Gardner XL    211 hazemeter in accordance with ASTM D-1003-61.-   (ii) Heat-seal strength of the heat-sealable layer to itself is    measured by positioning together and heating the heat-sealable    (second) layers of two samples of polyester film at 140° C. for one    second under a pressure of 275 kPa (40 psi). The sealed film is    cooled to room temperature, and the sealed composite cut into 25 mm    wide strips. The heat-seal strength is determined by measuring the    force required under linear tension per unit width of seal to peel    the layers of the film apart at a constant speed of 4.23 mm/second.-   (iii) Heat-seal strength to a standard APET/CPET tray is measured by    the following procedure. The coated film is sealed, by means of the    coating layer, to a standard APET/CPET tray (manufactured by    Faerchplast) using a Microseal PA 201 (Packaging Automation Ltd,    England) tray sealer at a temperature of 180° C., and pressure of 80    psi for one second. Strips (25 mm) of the sealed film and tray are    cut out at 90° to the seal, and the load required to pull the seal    apart measured using an Instron Model 4301 operating at a crosshead    speed of 0.2 m.min⁻¹. The procedure is repeated and a mean value of    5 results calculated.-   (iv) Delamination Bond Strength is measured by the following    procedure. Using a straight edge and a calibrated sample cutter (25    mm+\−0.5 mm) five strips of laminate of minimum length 100 mm are    cut. Peel between the laminated layers is initiated at one end of    each sample and the laminates peeled apart over a distance of    approximately 40 mm in length. Each sample in turn is tested using    an Instron model 4464 materials test machine, using pneumatic action    grips with rubber jaw faces. Crosshead speed is 50 mm\min⁻¹. The    samples are inserted into the Instron jaws with one layer clamped in    the fixed jaws and the other half in the moving jaws ensuring that    an equal amount of each layer is held in each jaw to allow the    laminate to be pulled apart evenly. The equipment records the mean    peel strength of each sample between 10 mm and 50 mm and the bond    strength of the laminate is quoted as an average of 5 samples in    g\25 mm.-   (v) Water vapour transmission rates are measured using a Lyssy Water    Vapour Permeation Tester model number L80 4000J. Test samples are    prepared by cutting around a template (100×110 mm). Once cut to size    samples are sealed into a standard Lyssy sample holder card by    placing in a hydraulic press for approximately 5 minutes at a    pressure of 2 tonnes.    -   Prior to testing the equipment is calibrated using standard 19        micron and 36 micron PET film (Melinex® S; DuPont Teijin Films;        m.p. 248° C.; relative density at 23° C. of 1.40). In order to        keep the conditioning and measuring chambers apart an        impermeable foil laminate is used in between testing of samples.        During sample changeover the test specimen and the foil layer        are placed on top of each other to ensure that at no time the        measuring chamber is directly exposed to the conditioning        chamber.    -   Testing is carried out at a temperature of 38° C. and at 90%        relative humidity. Equilibrium is reached when consecutive        results change by less than 2% indicated by an asterisk on the        equipment printout. Water vapour transmission rate quoted in        g/m²/day is recorded when 5 consecutive asterisks are seen on        the printout.-   (vi) Elongation At Break is measured according to test method ASTM    D882. Using a straight edge and a calibrated sample cutter (25    mm+\−0.5 mm) five strips (100 mm in length) of the film long are cut    along machine direction. Each sample is tested using an Instron    model 3111 materials test machine, using pneumatic action grips with    rubber jaw faces. Temperature and relative humidity (23° C., 50% rh)    are controlled. The crosshead speed (rate of separation) is 25    mm.min⁻¹. The strain rate is 50%. It is calculated by dividing the    rate of separation by the initial distance between grips (sample    length). The equipment records the elongation at break of each    sample. The Elongation At Break (ε_(B) (%)) is defined as:    ε_(B) (%)=(extension at break/L ₀)×100    -   where L₀ is the original length of the sample between grips.

The invention is further illustrated by the following examples. It willbe appreciated that the examples are for illustrative purposes only andare not intended to limit the invention as described above. Modificationof detail may be made without departing from the scope of the invention.

EXPERIMENTAL

Mono-Layer Films

A series of copolyesters were made using terephthalic acid, ethyleneglycol and a third component, as set out in Table 1. The mol % of thethird component (either a poly(alkylene oxide) glycol or a seconddicarboxylic acid) is given in Table 1 as the mol % of the diol and/ordiacid fraction in the final copolyester, respectively. The copolyesterswere made using conventional techniques either via thetrans-esterification route (using the dimethyl ester of terephthalicacid) or via direct esterification.

These copolyesters were then extruded through a film-forming die on to awater-cooled rotating, quenching drum at various line speeds to yield anamorphous cast composite extrudate. The cast extrudate was heated to atemperature of between about 70 and 105° C. (the actual temperature usedbeing dependent on the polymeric material in that a lower temperaturewas used for a softer polymer (i.e. one having a lower Tg)) and thenstretched longitudinally at a forward draw ratio of about 3:1. Thepolymeric film was passed into a stenter oven at a temperature of about100° C., where the sheet was stretched in the sideways direction toapproximately 4 times its original dimensions. The biaxially stretchedpolymeric film was optionally heat-set at the temperatures given inTable 1 below, which also shows the final thicknesses and WVTR data ofthe films. The films were clear; for example the film of Example 1 had ahaze of 1.5%. For the purposes of comparison, a standard PET monolayer(19 μm) film has a WVTR of 20 g/m²/day.

TABLE 1 Polymer Additional Monomer Thickness Process WVTR SampleIdentity Mol % (μm) Conditions (g/m²/day) 1 Terethane ® 22 16 (a) 169650 23 131 2 Terethane ® 22 14 (a) 150 1000 17 (a) 128 23 (b) 65 3 PEG1000 37 12 (a) 440 15 (a) 293 22 (a) 121 22 (c) 164 4 PEG 4000 28 11 (a)296 15 (a) 169 20 (a) 122 10 (b) 213 12 (b) 180 5 PEG 4000 38 10 (a) 52017 (b) 308 6 PEG 8000 27 10 (a) 187 12 (b) 117 7 PEG 10000 24 11 (a) 18217 (a) 129 11 (b) 110 8 PPG 32 10 (a) 240 17 (b) 135 9 Azelaic Acid 9 &27 11 (a) 515 & PEG4000 19 (a) 294 11 (b) 520 18 (c) 312Process Conditions in Table 1:

-   -   (a) no heat-setting;    -   (b) heat set at 220° C. for 15 seconds;    -   (c) heat set at 180° C. for 15 seconds.        Polymer Grades used in the polymers used in Table 1:    -   The Terethane® copolymers are poly(tetramethylene oxide) glycols        obtained from DuPont, US;    -   The PEG materials were obtained from Aldrich (Gillingham, UK);    -   The PPG materials were obtained from Aldrich (Gillingham, UK).

The data in Table 1 demonstrate the utility of poly(alkylene oxide)glycols in imparting breathability to a PET polymeric film.

Composite Films

Composite films were also manufactured by coextrusion in which the firstlayer was a breathable copolyester as described hereinabove and thesecond layer was a breathable heat-sealable polymeric layer. Thecopolymeric materials were derived by reacting terephthalic acid,ethylene glycol and a third and optionally a fourth component to formcopolymers P1, P2, P3, P4, HS1 and HS2 as set below. The mol % value isthat measured in the final polymer.

-   P1: PEG 1000 at a level of 26% of the glycol fraction-   P2: PEG 1000 at a level of 37% of the glycol fraction-   P3: PEG 3350 at a level of 31% of the glycol fraction-   P4: PEG 3350 at a level of 33.5% of the glycol fraction; and sodium    sulpho-isophthalic acid at a level of 1.4% of the acid fraction-   HS1: isophthalic acid at a level of 18% of the acid fraction-   HS2: PEG 400 at a level of 10% of the glycol fraction; and    isophthalic acid at a level of 18% of the acid fraction

The copolyesters were coextuded using separate streams supplied fromseparate extruders, to a single channel coextrusion assembly. Thepolymer layers were extruded through a film-forming die on to awater-cooled rotating, quenching drum at various line speeds to yield anamorphous cast composite extrudate. The cast extrudate was heated to atemperature in the range of about 50 to 80° C. and then stretchedlongitudinally at a forward draw ratio of about 3:1. The polymeric filmwas passed into a stenter oven at a temperature of about 110° C., wherethe sheet was stretched in the sideways direction to approximately 4times its original dimensions, and then heat-set at temperatures between210 and 225° C. The final thickness of the film was 23 μm, in which thesecond (heat-sealable) layer was 10-15% of the thickness of the firstlayer. The films were clear; Example 6 for instance had a haze of 4.5%.The WVTR of the films are given in Table 2.

TABLE 2 Film COPOLYESTERS WVTR Example 1^(st) Layer 2^(nd) Layer(g/m²/day) Control PET HS1 22 1 P1 HS1 61 2 P1 HS2 75 3 P2 HS1 84 4 P2HS2 111 5 P3 HS2 94 6 P4 HS2 120

The invention claimed is:
 1. A breathable, heat-sealable composite filmcomprising first and second layers of polymeric material wherein: (i)the polymeric material of the first layer comprises a copolyestercomprising monomeric units derived from one or more diol(s); one or moredicarboxylic acid(s), wherein at least one of said one or moredicarboxylic acids is an aromatic acid; and one or more poly(alkyleneoxide)glycol(s); (ii) the second layer is a heat-sealable polymericlayer; (iii) the composite film has an elongation to break of less than250%; and (iv) the poly(alkylene oxide) glycol of the first layerconstitutes in the range of 10 to 40 mol % of all diol and glycolmonomeric units of the first layer; wherein the polymeric material ofthe first layer comprises greater than 50% of the copolyester by weight,relative to the total amount of polymeric material in the first layer,and wherein the composite film is uniaxially or biaxially oriented. 2.The film according to claim 1 wherein the film is permeable to gaseouswater.
 3. The film according to claim 1 wherein the poly(alkylene oxide)glycol is selected from the group consisting of polyethylene glycol(PEG), polypropylene glycol (PPG) and poly(tetramethylene oxide) glycol(PTMO).
 4. The film according to claim 1 wherein the poly(alkyleneoxide) glycol is polyethylene glycol.
 5. The film according to claim 1wherein the average molecular weight of the poly(alkylene oxide) glycolis from about 400 to about
 10000. 6. The film according to claim 1wherein the average molecular weight of the poly(alkylene oxide) glycolis from about 400 to about
 4500. 7. The film according to claim 1wherein the copolyester of the first layer comprises terephthalic acid.8. The film according to claim 1 wherein the copolyester of the firstlayer comprises an aliphatic diol.
 9. The film according to claim 1wherein the copolyester of the first layer comprises ethylene glycol.10. The film according to claim 1 wherein said one or more dicarboxylicacid(s) comprises a first aromatic dicarboxylic acid and a seconddicarboxylic acid.
 11. The film according to claim 10 wherein said firstaromatic dicarboxylic acid is terephthalic acid.
 12. The film accordingto claim 11 wherein said second dicarboxylic acid is a sulphonatedaromatic dicarboxylic acid.
 13. The film according to claim 12 whereinsaid second dicarboxylic acid is sodium sulpho-isophthalic acid.
 14. Thefilm according to claim 11 wherein said second dicarboxylic acid is analiphatic dicarboxylic acid.
 15. The film according to claim 14 whereinsaid second dicarboxylic acid is adipic acid or azelaic acid.
 16. Thefilm according to claim 10 wherein the first dicarboxylic acidconstitutes from 50 to 100 mol % of all dicarboxylic acid monomericunits, and the second dicarboxylic acid constitutes from 0 to 50 mol %of all dicarboxylic acid monomeric units.
 17. The film according toclaim 1 wherein the thickness of the first layer is in the range of fromabout 0.5 to about 300 μm.
 18. The film according to claim 1 wherein theheat-sealable second layer is derived from a first dicarboxylic acid, afirst low molecular weight diol, and either a second dicarboxylic acidor a second low molecular weight diol.
 19. The film according to claim18 wherein the heat-sealable second layer is a copolyester derived fromethylene glycol, terephthalic acid and isophthalic acid.
 20. The filmaccording to claim 18 wherein the heat-sealable second layer is acopolyester derived from terephthalic acid, ethylene glycol and1,4-cyclohexanedimethanol.
 21. The film according to claim 1 wherein thepolymeric material of the heat-sealable second layer is derived from afirst aromatic dicarboxylic acid, a low molecular weight aliphatic diol,a poly(alkylene oxide) glycol, and a further dicarboxylic acid whichimparts heat-sealability.
 22. The film according to claim 21 whereinsaid first aromatic dicarboxylic acid is terephthalic acid, and/or saidlow molecular weight aliphatic diol is ethylene glycol, and/or saidfurther dicarboxylic acid which imparts heat-sealability is isophthalicacid.
 23. The film according to claim 21 wherein said first dicarboxylicacid constitutes from 50 to 90 mol % of all dicarboxylic acid monomericunits.
 24. The film according to claim 21 wherein said low molecularweight diol constitutes from 60 to 90 mol % of all diol and glycolmonomeric units.
 25. The film according to claims 21 wherein thepoly(alkylene oxide) glycol is poly(ethylene oxide) glycol.
 26. The filmaccording to claim 1 wherein the composite film is a coextrudedcomposite film.
 27. The film according to claim 1 wherein theheat-sealable second layer comprises a copolyester derived from anaromatic dicarboxylic acid and an aliphatic dicarboxylic acid.
 28. Thefilm according to claim 1 wherein the heat-sealable second layercomprises ethylene vinyl acetate (EVA).
 29. The film according to claim1 wherein the thickness of the second layer is in the range of fromabout 0.05 to about 20 μm.
 30. The film according to claim 1 wherein thecomposite film exhibits a water vapour transmission rate (WVTR) of atleast 60 g/m²/day.
 31. The film according to claim 1 wherein thecopolyester of the first layer has a glass transition temperature (Tg)of at least 0° C. and no more than 60° C.
 32. A breathable,heat-sealable composite film comprising first and second layers ofpolymeric material wherein: (i) the polymeric material of the firstlayer comprises a copolyester comprising monomeric units derived fromone or more diol(s); one or more dicarboxylic acid(s), wherein at leastone of said one or more dicarboxylic acids is an aromatic acid; and oneor more poly(alkylene oxide)glycol(s); (ii) the second layer is aheat-sealable polymeric layer; (iii) the composite film has anelongation to break of less than 250%; and (iv) the poly(alkylene oxide)glycol of the first layer constitutes in the range of 10 to 40 mol % ofall diol and glycol monomeric units of the first layer; wherein thepolymeric material of the first layer comprises greater than 50% of thecopolyester by weight, relative to the total amount of polymericmaterial in the first layer, and wherein the composite film exhibits ahaze of less than 6%.
 33. A method of packaging cut plants, comprisingheat-sealing the plants within a container comprising a breathable,heat-sealable composite film comprising first and second layers ofpolymeric material wherein: (i) the polymeric material of the firstlayer comprises a copolyester comprising monomeric units derived fromone or more diol(s); one or more dicarboxylic acid(s), wherein at leastone of said one or more dicarboxylic acids is an aromatic acid; and oneor more poly(alkylene oxide)glycol(s); (ii) the second layer is aheat-sealable polymeric layer; (iii) the composite film has anelongation to break of less than 250%; and (iv) the poly(alkylene oxide)glycol of the first layer constitutes in the range of 10 to 40 mol % ofall diol and glycol monomeric units of the first layer; wherein thepolymeric material of the first layer comprises greater than 50% of thecopolyester by weight, relative to the total amount of polymericmaterial in the first layer.
 34. The method according to claim 33wherein the container further comprises a receptacle for the cut plants,wherein the film forms a lid for the receptacle, and wherein the step ofheat-sealing comprises heat-sealing the film to the receptacle.
 35. Asealed container comprising a receptacle containing cut plant(s), and alid formed from a breathable, heat-sealable composite film comprisingfirst and second layers of polymeric material wherein: (i) the polymericmaterial of the first layer comprises a copolyester comprising monomericunits derived from one or more diol(s); one or more dicarboxylicacid(s), wherein at least one of said one or more dicarboxylic acids isan aromatic acid; and one or more poly(alkylene oxide)glycol(s); (ii)the second layer is a heat-sealable polymeric layer; (iii) the compositefilm has an elongation to break of less than 250%; and (iv) thepoly(alkylene oxide) glycol of the first layer constitutes in the rangeof 10 to 40 mol % of all diol and glycol monomeric units of the firstlayer; wherein the polymeric material of the first layer comprisesgreater than 50% of the copolyester by weight, relative to the totalamount of polymeric material in the first layer.